1. Technical Field of the Invention
The present invention relates generally to a spark plug which may be employed in automotive engines, and more particularly to such a spark plug with noble metal chips joined to a center and a ground electrode by unique laser welding for ensuring higher durability of the spark plug and ignitability of a gaseous fuel and a fabrication method therefor.
2. Background Art
Typical spark plugs for automotive engines or gas engines are equipped with a center electrode and a ground electrode. The center electrode is disposed within a metal shell and has a tip exposed outside the metal shell. The ground electrode is joined at one end thereof to the metal shell and bent to have the other end thereof face the center electrode through a spark gap.
Recently, in order to improve the durability of the spark plugs and ignitability of fuel, noble metal chips made of Pt (Platinum) or Ir (Iridium) have been used which are laser-welded to surfaces of the center and ground electrodes opposed to each other through the spark gap.
Japanese Patent First Publication No. 2001-135456 teaches conventional laser welding to joint the noble metal chips to the center and ground electrodes. The laser welding is achieved by emitting laser beams around entire circumferences of interfaces between the noble metal chips and the center and ground electrodes at orientations which do not optically interfere with the metal shell.
The above laser welding will be described below in detail with reference to FIG. 27.
A noble metal chip 45 is welded to a ground electrode 40. The ground electrode 40 is welded at an end thereof to a metal shell (not shown).
The weld of the noble metal chip 45 to the ground electrode 40 is achieved by emitting laser beams LZ around an interface between the noble metal chip 45 and a side surface 43 of the ground electrode 40 to form fused portions 44 (also called weld nuggets).
In the drawing, θL represents the angle between the side surface 43 and orientation of radiation of each of the laser beams LZ (will also be referred to as a radiation angle below) In the following discussion, LZ will also represents a laser radiation path along which the laser beams LZ are radiated.
The radiation of the laser beams LZ is performed after the ground electrode 40 is welded to the metal shell, but before bent.
In a case where the metal chip is located on the left side of the noble metal chip 45, as viewed in the drawing, elimination of interference of the laser beams LZ with the metal shell requires increasing the radiation angle θL which causes the laser beams LZ to travel beyond the metal shell toward the side surface 43. The increased radiation angle θL, however, will result in a decrease in depth DW of a portion of the noble metal chip 45 which is fused by the laser beam LZ into the ground electrode 40 in a radius direction of the noble metal chip 45, thus leading to an undesirable increase in unfused area in the interface between the noble metal chip 45 and the ground electrode 40.
In modern engines, a combustible atmosphere is elevated in temperature for increasing an output power and reducing a fuel consumption and exhaust emissions. In this type of engine, a park plug is subjected to an intense heat, so that the temperature of center and ground electrodes is increased greatly. The electrodes, therefore, undergo a thermal stress and oxidation, which may cause the noble metal chips to be removed from the center and ground electrodes. Particularly, such a problem is exacerbated in the ground electrode because it is exposed to a combustion chamber of the engine more than the center electrode.
The noble metal chip 45 of the spark plug, as disclosed in the above publication, has a relatively shorter length t projecting from the side surface 43. A decrease in the length t may cause the fused portion 44 to reach a spark discharging surface 45a, thus causing the fused portions 44 to be worn earlier than the noble metal chip 45, which, in the worst case, results in separation of the noble metal chip 45 from the ground electrode 40.
In order to avoid the above problem, US2002/01105254 A1 teaches laser welding techniques of decreasing the radiation angle θL to increase the melted depth DW of the noble metal chip 45. Specifically, the radiation angle θL is decreased to set a melt angle less than 60°. The melt angle is the angle which a line extending through the fused portion 44 along a maximum depth of the fused portion 44 makes with the side surface 43 of the ground electrode 40. This results in a decrease in size of an unfused portion in the interface between the noble metal chip 45 and the ground electrode 40, thereby ensuring the reliability of a joint between the noble metal chip 45 and the ground electrode 40 in a high temperature combustible atmosphere.
The laser welding, as taught in the latter publication, has the drawback in that the metal shell will be an obstruction to the traveling of the laser beams, thus resulting in a difficulty in welding the entire circumference of the interface between the noble metal chip 45 and the ground electrode 40. The spark plug is, thus, assembled in a manner wherein the noble metal chip 45 is welded to the side surface 43 of the ground electrode 40, after which the ground electrode 40 is welded to the metal shell. Such assembling process, however, results in decreased productivity of the spark plug. Specifically, the noble metal chip 45 installed on the ground electrode 40 obstructs the welding of the ground electrode 40 to the metal shell, which will lead to an increase in production cost of the spark plug. In the worst case, when the ground electrode 40 is chucked, the noble metal chip 45 may be broken.